Well, after much discussion, and some help from some propane afficionados, I now have the propane slant six running GOOOOOD. First, too small of converter, so I plumbed in a second. Second, too much restriction in the fuel hose and fittings, so replaced with larger and more open ones, and starting to reset the timing curves.

This thing runs GOOD now. Still a rough, rump-rump idle, but any other condition it's smooth, and pulls to my programmed timing limited rpm (timing drops to 0 degrees) at 4000 without any observable drop on power. Once the engine gets "on cam" at about 1700 it climbs in torque to about 2300 and the truck actually feels peppy. It has WAY more power than it did with the 1bbl gasoline setup.

Just as an overview:

10.5:1 CR - by milling head well past .100 off.
Oversize stainless valves from enginbldr
hardened seats, all 12
custom ground Oregon Cams 260I / 248E .440 / 437
orginal exhaust manifold (for now)
super six intake
Impco 200 mixer
Ford EDIS ignition, crank trigger, uses chrysler 3.3 coil
Megajolt timing control
cold air intake
mild intake and exhaust bowl cleanup and some minor port smoothing
still wanting headers, dutras, or "something" to get rid of intake manifold heat.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Sounds like a nice system, and glad you're getting it sorted out.

0 deg timing seems like it cannot be optimal at higher RPM/load. You should need at least a little time for the mix to burn and reach max pressure around TDC (0 deg). I'm no expert on propane...

Lou

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Home of Slant6-powered fun machines

Lou, please read carefuly... the "0" degree timing is an rpm limiter. The timing starts to drop after 4000 and falls to 0 by 4100. I had it set to 3800 earlier and thought it might like going farther.

It really sounds funny, too, when it hits it.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Oops. Yes, thanks for clarifying. Slant on!

Lou

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Home of Slant6-powered fun machines

Well, after upsizing the feed hose to the carb and using less restrictive fittings... WOW! I had no idea this thing could go like it does. It never ran even faintly this good on gas.

You won't mistake it for a good 318 or bigger, but it certainly does NOT feel weak in any way. The 5000 lb truck feels strong, even on grades, against strong winds, too. It easily runs 65-70 without feeling like you're even pushing it. I wish I had this in a 3600 lb car. It would be downright fun to drive!

I max out the WOT timing at 24 degrees @ 3500, and it certainly doesn't seem to want any more. It's 21 degrees@ 2100 WOT and that seems good. Tried a couple degrees more and a couple less, and this seems as good as it gets. I'm going to eventually run this on a dyno and set it for max torque, at wot and very light throttle, for given speeds, for both light load and wot settings.

One more question: Will it help to add a 1 inch or taller spacer between the carb and 2 bbl intake? I was thinking of having the local machine shop build me one that blends the oval into the two holes (or cutting out the center of the intake), for a little more lower speed ram effect.

Hope I can afford some dutras or something soon. need to get the heat off the manifold.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Yeah, fuel is important.

I don't think the added plenum volume is going to help you, but you can try it. If you want to streamline things you can work the transition from the carburetor mounting into the plenum. With your dry fuel I don't think the two hole manifold helps and with a spacer I would definitely open it up.

In High School I had the heat pocket on the bottom of the intake manifold milled off and bolted a piece of 1/4" plate steel to the top of the exhaust manifold to seal it up. All to keep the intake cooler. Worked for sh!t in cool, damp weather with a 350CFM Holley, but it will get you closer to what you want.

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Joshua

How does your new fuel compare in dollars per mile for fuel?

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1973 Dodge Dart Custom. The slant is the perfect tool for yarding the queen mary.

Most performance LPG ( propane) guys in Oz prefer a larger plenum single plane intake or an EFI style setup on anything but a stocker. As for a slant six I guess it's certainly worth a try,I have a 318 stroker with a factory single plane intake and it definitely benefited from a spacer, but its also has an extra 30 cubes! It will be a very interesting trial. Propane is famous for its smooth efi quality idle, keep tuning you should get there. Definitely get rid of the intake heating, it's just robbing power. It sounds like you have a very nice setup,best of luck with the finishing touches, I hope you are as happy with your conversion as I am with my LPG setup!